The electrolyte is a critical component in lithium-ion batteries (LIBs) and must evolve with the changing landscape of cathode materials. For example, the next generation of cathode materials will require high-voltage operating conditions (4.5 V or greater vs. Li/Li+) in order to supply high capacity and high power demands. Nickel-rich layered oxide materials, such as LiNixMnyCozO2 (where x+y+z=1) (NMC), have shown tremendous promise as high energy density cathode materials and operate at voltages up to about 4.5 V vs. Li/Li+ (about 4.4 V is a NMC/graphite full cell); however, these materials suffer from electrolyte oxidation and transition metal dissolution when cycled repeatedly at this operating voltage. One approach to dealing with the high-voltage shortcomings of nickel-rich layered oxide materials is the inclusion of electrolyte additives to mitigate detrimental effects observed at high operating voltages. This is particularly the case in the evolving landscape of batteries for electric vehicles.
Electrolyte additives can be classified into several groups depending upon their function in the cell: negative electrode additives, positive electrode additives, hydrofluoric acid (HF) scavengers, over-charge protection (“shuttle”) additives, and the like. Additives that can employ two or more of these attributes are referred to as “multifunctional” and are uncommon. Commonly used additives such as vinylene carbonate (VC) and lithium bis(oxalate)borate (LiBOB) provide a more robust solid-electrolyte interphase (SEI) on both the negative and positive electrodes. However, due to the resistive nature of these surface films, the impedance of the cell can be significantly increased. Additives such as tris(trimethylsilyl)phosphite (TMSPi) are capable of reacting with HF in the electrolyte, but have limited ability to be functionalized.
There is an ongoing need for multifunctional additive materials for lithium-ion batteries that will be operated under relatively high-voltage conditions (i.e., greater than or equal to 4.5 V versus Li/Li, or greater than or equal to 4.4 V in a NMC/graphite full cell). The additive materials described herein address this need.